The
use of ultrasonic instrumentation has grown significantly over the past
20 years. Ultrasonic
therapy is less taxing on clinicians' musculature than hand
instrumentation and offers
the additional benefit of lavage. The ultrasonic scaler has proven
effective in calculus and biofilm removal, but does it offer
additional benefits? Some assert that the formation of pulsating
bubbles (cavitation) and acoustic microstreaming (fluid flow generated
by ultrasonic oscillations) created by ultrasonic scalers may have
bactericidal effects. The discussion of cavitation and the possibility
that it could disrupt dental biofilm began in 1984 when Dr. Walmsley's
paper "A Model System to Demonstrate the Role of
Cavitational Activity in Ultrasonic Scaling" was published in the Journal of Dental Research.1 Because no in vivo research has been published
on the topic, the debate over the role of cavitation and acoustic microstreaming continues. Dimensions of Dental Hygiene had
the opportunity to speak with Dr. Walmsley about his thoughts on the subject.

How does an ultrasonic scaler work?

The ultrasonic scaler's metal probe oscillates at high frequencies.
It is the energy contained in this movement that disrupts
microbial biofilms and fractures calculus deposits. This
impact with the tooth may also lead to damaging effects, but
these are avoided in the hands of well-trained operators who
understand how the instrument works. While the ultrasonic
instrument is operating, a flow of cooling water passes over the
tip, reducing heat that might otherwise be problematic for vital
teeth. As water flows across the ultrasonic tip, additional physical
phenomena are observable—including cavitation and
acoustic microstreaming.

What forces affect cavitation and acoustic
microstreaming in ultrasonic instrumentation?

As noted, cavitation is the formation of pulsating bubbles
that are powered by an ultrasonic field. When an ultrasound
wave passes through water, molecules are pushed closer
together and pulled apart in a split second. If the movement of
these sound waves is high, as is the case in the frequencies
used in dentistry, the micromillimeter-sized bubbles expand
and collapse violently. The subsequent energy is released as a
shock wave, heat, and/or abrupt changes in nearby hydraulic
pressures. Though a single bubble's energy is quickly dissipated,
within the coolant there will be thousands, if not millions,
of other bubbles produced that will then collapse—creating the appearance of pulsation.

Acoustic microstreaming is another
energy released around ultrasonic devices.
This phenomenon is characterized by the
movement of small currents in the water
(Figure 1). Micro streaming commonly occurs
around oscillating objects, such as cavitation
bubbles or the scaler tip. These currents produce
shear forces that are strong enough to
break up clumps of bacteria but not powerful
enough to break down bacterial cell walls.2 These forces have been shown to cleave flagella
or other protuberances off bacterial
walls. They are also able to break up colonies
of bacteria and disrupt biofilm.3

What does cavitation look like?

It is possible to depict cavitation around ultrasonic instruments using various laboratory
and photographic techniques. The intense
energy released from within the bubbles can break down water or other chemicals.4 Luminol
is a chemical that glows in response to this
cavitation effect, allowing us to visualize what
is taking place. Figure 2 illustrates that cavitation
is most intense at the bend of a typical
ultrasonic insert/tip (UIT) design. This image
was taken with the instrument working freely
within a water chamber. However, when the
ultrasonic scaler contacts a solid surface, such
as a tooth, the cavitation still takes place,5 although more studies are needed to verify
the occurence and degree of cavitation. Further
research is required to determine
whether cavitation is occurring at the contact
area between the tip and the tooth while it is
moving within the periodontal pocket. It is
also not known how or if the size, shape, or
style of the UIT might affect this process.

Early laboratory work demonstrated the
removal of plaque from teeth by an action
within the water that was not necessarily
associated with direct contact of the tip
against the tooth.6,7 This was attributed to
cavitational activity and acoustic microstreaming,
as both phenomena are interrelated.

What does the evidence say about
the effects of cavitation and microstreaming?

Evidence of these effects by cavitation and
acoustic microstreaming have only been
demonstrated with in vitro research. More clinical
research is needed because the contribution
of such forces is not fully understood.
Additional studies (including in vivo research)
are necessary to understand possible clinical
benefits. The attached biofilm may be disrupted
by the streaming forces, which in turn
allows for tissue healing. A greater evidence
base is required with more randomized, controlled
clinical trials to translate laboratory
results into clinically-relevant outcomes.

What might the future hold in ultrasonic
scaler technology?

Research should focus on whether and to
what extent cavitation occurs in the periodontal
pocket, and whether cavitation plays
a role in removing biofilm from deep within
periodontal pockets. It may be that the combination
of the tip contact and the movement
of fluid is what disrupts and dislodges the biofilm. A call was made at a recent international
workshop for research to be conducted
on the occurrence of cavitation in vivo and, if demonstrated, how it can be
maximized to ensure that the biophysical
phenomenon within the periodontal pocket
is used to good effect.8 Based on laboratory
studies, there is strong evidence that cavitation
occurs. However, it is a difficult phenomenon
to study clinically. New and
improved imaging techniques will allow us
to observe what is happening when using
ultrasonic scalers in deep periodontal pockets.
If cavitation and acoustic microstreaming
do improve clinical outcomes, we may
be able to develop ultrasonic technologies to
maximize the potential advantage of these
phenomena.

Acknowledgment

Dr. Walmsley wishes to thank the United
Kingdom Engineering and Physical Sciences
Research Council for grant numbers
EP/C536894 and EP/F020090, in addition
to Simon Lea, MSc, PhD, and Gareth Price,
PhD, for support of this research.

The views expressed in this interview are
those of A. Damien Walmsley, BDS, MSc, PhD

A. Damien Walmsley, BDS, MSc, PhD,
is a professor of restorative dentistry at the University of Birmingham
School of Dentistry and a consultant in restorative dentistry to South
Birmingham Community Health Trust, both in the United Kingdom. He also
serves as scientific advisor for the British Dental Association.
Walmsley's research interests include the use of ultrasonics in
dentistry, implant overdentures, and e-learning.